1. Field of the Invention
The present invention relates to a power conversion circuit connected to an AC power system and DC power system and capable of bidirectionally converting AC power and DC power and, more particularly, to a power conversion circuit control apparatus capable of reducing overcurrent of the power conversion circuit when the voltage of the AC power system fluctuates, thereby rapidly reactivating and operating the power conversion circuit even when the current cannot be controlled due to the overcurrent.
2. Description of the Related Art
Conventionally, a power conversion circuit which is connected to power systems, bidirectionally converts AC power and DC power, and modulates the pulse width of the DC voltage can control the active power and reactive power of the AC system, and is used in power interchange of the power systems, and stabilization and fluctuation compensation of the voltage (patent references 1 and 2, and non-patent reference 1).
If a system failure occurs, as disclosed in patent reference 1, a conventional power conversion circuit control apparatus stops controllable elements of the power converter, and continues the operation for a minimum time period necessary to stop the system current. After that, the control apparatus operates the controllable elements of the power converter to allow the current to flow.
In another conventional power conversion circuit control apparatus, as disclosed in patent reference 2, the output voltage of a power converter is matched with the magnitude and phase of the voltage of a power system before the power converter is connected to the power system.
In still another conventional power conversion circuit control apparatus, as disclosed in non-patent reference 1, after a polyphase AC voltage is decomposed into a positive-phase component and negative-phase component, a power converter is controlled by calculating the magnitude and phase of a voltage output command of each phase of the power converter (FIG. 6 in non-patent reference 1). The positive-phase component and negative-phase component are separated by converting the polyphase AC voltage into an orthogonal two-phase AC voltage, and performing calculations from a ¼-period delayed signal (FIG. 2 in non-patent reference 1).
Patent reference 1: Jpn. Pat. Appln. KOKAI Publication No. 6-175741, page 5 (FIGS. 1 and 2)
Patent reference 2: Jpn. Pat. Appln. KOKAI Publication No. 54-82649 (FIG. 4)
Non-patent reference 1: Y. Jiang, A. Ekstrom, Applying PWM to Control Overcurrents at Unbalanced Faults of Forced-Commutated VSCs Used as Static Var Compensators”, IEEE Transactions on Power Delivery, Vol. 12, No. 1, pp. 273-278, January 1997, FIGS. 2 and 6
In the conventional power converter control apparatus disclosed in patent reference 1, the failure of the power system may continue when the system current is allowed to flow again. Accordingly, the power converter operates the controllable element while the polyphase AC voltage of the power system is unbalanced. Since this increases the difference between the power converter voltage and power system voltage, an overcurrent flows through the power converter to stop the controllable element again. The repetition of this delays the restart of the operation.
In the conventional power converter control apparatus disclosed in patent reference 2, if the polyphase AC voltage of the power system is unbalanced, the magnitude and phase of the output voltage command form a signal which pulsates at a period twice that of the frequency of the power system. This makes the power converter output voltage different from the power system voltage. In the conventional power conversion circuit control apparatus disclosed in FIG. 6 of non-patent reference 1, the polyphase AC voltage is separated into a positive-phase component and negative-phase component, and this produces a delay of at least a ¼ period. If the period is ¼ or less, therefore, no correct power system voltage can be obtained, and a voltage difference is produced between the power converter and power system. As a consequence, an overcurrent may flow through the power converter.